Recording/reading apparatus for inscribable record carrier and its manufacture

ABSTRACT

A record carrier (1) in the form of a disc-shaped carrier provided with a radiation-sensitive layer (6) having a servo track (4). The servo track produces track modulation by having a radial wobble, or lateral periodic deviation whose frequency is modulated with a position-information signal (FIG. 2). Apparatus (FIG. 8) is presented which forms the track pattern during manufacture of the record carrier (1). When an information signal (Vi) is recorded on the record carrier (1) and the recorded signal is read by recording and/or read apparatus (FIG. 4), the position-information signal (FIG. 2) is recovered by an FM demodulator device (60) from variations in the scanning beam (55) which are propduced by the track modulation. Moveover, a clock signal for the purpose of scanning-velocity control is recovered from this variation in the scanning beam. Furthermore, embodiments of the record carrier highly suitable for recording EFM-modulated signals are described.

BACKGROUND OF THE INVENTION

This invention relates to an optically readable record carrier of theinscribable type having a recording layer intended for recording aninformation pattern of optically detectable recording marks wherein aservo track is provided exhibiting a periodic track modulation which canbe distinguished from the information pattern.

The invention additionally relates to apparatus for manufacturing arecord carrier including an optical system for scanning aradiation-sensitive layer of the carrier along a path corresponding tothe servo track to be formed and a deflection device for deflecting theradiation beam so that the position of incidence of the beam on theradiation-sensitive layer is deflected in a direction perpendicular tothe scanning direction in conformity with a control signal applied tothe deflection device.

The invention also relates to apparatus for recording information on arecord carrier including an arrangement for recording a pattern ofrecording marks representing the information in the servo track. Thearrangement includes: a scanner for scanning the servo track with aradiation beam so that the radiation beam reflected or transmitted bythe record carrier is modulated by the track modulation: a detector fordetecting the reflected or transmitted radiation beam: and a circuit forderiving a clock signal whose frequency is determined by the trackmodulation from the radiation received by the detector.

The invention further relates to apparatus for reading a record carrierin whose servo track an information signal is recorded as a pattern ofrecording marks. The apparatus includes: a scanning device for scanningthe servo track with substantially constant velocity by means of aradiation beam: the radiation beam reflected or transmitted by therecord carrier being modulated by the track modulation and the patternof recording marks: a detector for detecting the reflected ortransmitted radiation beam: a circuit for deriving an information signalrepresenting the recorded information from the radiation detected by thedetector, and a circuit for deriving a clock signal whose frequency isdictated by the track modulation from the radiation detected by thedetector.

This type of record carrier including associated apparatus are knownfrom German patent document Offenlegungsschrift No. 3100421.

This conventional record carrier has a spiral servo track exhibiting atrack modulation of constant frequency. As the spiral servo track isscanned by a radiation beam during reading and/or recording, this trackmodulation modulates the radiation beam. This modulated beam is detectedand from the modulation a clock signal is derived which is utilized forcontrolling the recording and/or reading process.

The servo track is divided into information-recording areas betweenwhich synchronization areas are interposed. The information-recordingareas are intended for recording the information. The synchronizationareas contain position information in the form of the address of theadjacent information-recording area. During scanning the positioninformation in the synchronization areas, it may appear possible toderive from the reflected radiation beam the identity of the portion ofthe record carrier that is being scanned. This would enable a specificportion of the disc to be located rapidly and accurately.

However, the conventional record carrier has the disadvantage that theinformation-recording areas are constantly interrupted bysynchronization areas. This is a drawback, in particular whenEFM-encoded information is recorded on the record carrier. This isbecause such a recording method requires an uninterruptedinformation-recording area.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique that isbetter adapted to record EFM-encoded signals and which during scanningmakes it possible to derive from the light beam reflected by the recordcarrier, the identity of the portion of the disc that is being scanned.

In accordance with that one aspect of the invention, a record carrier ofthis type is characterized in that the frequency of the track modulationis modulated in conformity with a position-information signal comprisingposition code signals which alternate with position-synchronizationsignals.

In accordance with another aspect of the invention, apparatus formanufacturing the record carrier includes a signal generator forgenerating a periodic signal which functions as the control signal andwhose frequency is modulated in conformity with a position-informationsignal having position-code signals alternating withposition-synchronization signals.

In accordance with a further aspect of the invention, apparatus forrecording information, comprises an FM demodulation circuit forrecovering the position-information signal from the clock signal andmeans for separating the position-code signals and theposition-synchronization signals.

In accordance with a still further aspect of the invention, apparatusfor reading information, comprises an FM demodulation circuit forrecovering the position-information signal from the clock signal andmeans for separating the position-code signals and theposition-synchronization signals.

When the track is scanned during information recording or reading, it isthus possible to recover the position-information signal from thescanning-beam modulation produced by the track modulation. As a resultof the insertion of position-synchronization signals, the position-codesignals representing the instantaneous scanning position can then berecovered simply.

Since the track modulation is situated in the information-recordingarea, the information-recording area need no longer be interrupted bysynchronization areas. Moreover, the center frequency of the modulationof the reflected radiation beam produced by the track modulation can beutilized for measuring the scanning velocity for the purpose of scanningvelocity control. For such a velocity control, it is advantageous toutilize an embodiment of the record carrier which is characterized inthat the position-code signal is a biphase-mark modulated signal, theposition-synchronization signal having a waveform which differs from thebiphase-mark-modulated signal. This is because a biphase-mark-modulatedsignal has a frequency spectrum which contains virtually nolow-frequency components, so that the velocity control, which respondsmainly to low-frequency disturbances, is hardly affected by the FM trackmodulation.

A further illustrative embodiment of the record carrier is characterizedin that the width of the servo track is between 0.4×10⁻⁶ meters and1.25×10⁻⁶ meters, the track modulation being in the form of a trackwobble, or lateral periodic deviation, having an amplitude which issubstantially equal to 30×10⁻⁹ meters. This embodiment of the recordcarrier has the advantage that the track modulation can be formed simplyduring the mastering process by giving the scanning; beam used forrecording an excursion in a direction perpendicular to the scanningdirection. Moreover, for a wobble amplitude of approximately 30×(10⁻⁹)meters the effect of the track wobble on the recording process is foundto be substantially nil. Indeed, the deviations of the scanning-beamposition relative to the center of the track during scanning arenegligibly small. Moreover, for such a low amplitude the minimaldistance between two adjacent servo tracks does not increasesignificantly. On the other hand, such a low amplitude is found to beamply sufficient for a reliable recovery of the position-informationsignal.

If it is desirable to record EFM signals in conformity with theconventional CD Audio standard, it is advantageous to use anillustrative embodiment of the record carrier which is characterized inthat the average period of the track modulation is between 54×10⁻⁶meters and 64×10⁻⁶ meters, the distance between the starting positionsof the track portions which are modulated in conformity with thesynchronization signal corresponding to 294 times the average period ofthe track modulation.

This embodiment provides velocity control in such a way that the centerfrequency of the scanning-beam modulation produced by the trackmodulation is maintained equal to a reference frequency of 22.05 kHz ata scanning velocity of 1.2-1.4 meters per second which is customary forrecording EFM signals. The bit rate of an EFM signal (4.3218 MHz) is anintegral multiple of 22.05 kHz, so that the reference frequency can bederived simply from the EFM bit rates by means of frequency coding. Asthe distance between track portions modulated by the synchronizationsignals is 294 times the average period of the track modulation therepetition rate of the recovered synchronization signals is 75 Hz, whichis exactly equal to the repetition rate of the subcode-synchronizationsignals contained in the standard EFM signal This provides a simplesynchronization between the position-synchronization signals derivedfrom the track modulation and the process of recording the EFM signals.

A further illustrative embodiment of the record carrier is characterizedin that the position-code signal indicates the time needed at thenominal scanning velocity to cover the distance between the beginning ofthe track and the position where the track exhibits the track modulationcorresponding to the position-code signal.

This embodiment has the advantage that the position information providedby the position-code signal is of the same type as the positioninformation provided by the absolute-time codes in the EFM signal,enabling a simple control system for the recording and read apparatus tobe obtained.

BRIEF DESCRIPTION OF THE INVENTION

Further embodiments and the advantages thereof will now be described inmore detail, by way of example, with reference to FIGS. 1 to 12, ofwhich

FIGS. 1a -1d show embodiments of the record carrier in accordance withthe invention,

FIG. 2 shows a position-information signal,

FIG. 3 shows a suitable format for the position-information codes,

FIG. 4 shows an embodiment of a recording and/or read apparatus inaccordance with the invention,

FIGS. 5 and 12 are flow-charts of programs for a microcomputer utilizedin the recording and/or read apparatus,

FIG. 6 shows an example of a demodulation circuit for use in therecording and/or read apparatus,

FIG. 7 shows a track portion formed with a pattern of recording marks toa highly enlarged scale,

FIG. 8 shows an example of an apparatus for manufacturing a recordcarrier by means of the method in accordance with the invention,

FIG. 9 shows an example of a modulation circuit for use in the apparatusshown in FIG. 8,

FIG. 10 shows a number of signals appearing in the modulator circuit asa function of time t, and

FIG. 11 illustrates the position of the time synchronization signals ofthe recorded signal relative to the prerecorded position-synchronizationsignals in the servo track.

DETAILED DESCRIPTION OF THE INVENTION

The illustrative embodiments of the invention described hereinafter areparticularly suitable for recording EFM signals in conformity with theCD-Audio or CD-ROM standard. However, it is to be noted that the scopeof the invention is not limited to these illustrative embodiments.

Before the embodiments are described a brief description will bepresented of those characteristics of the EFM signal which are relevantfor a correct understanding of the invention. The EFM signal comprisessubcode frames of 98 EFM frames each. Each EFM frame comprises 588 EFMchannel bits. The first 24 bits of these 588 EFM channel bits areemployed for a frame synchronization code, which has a pattern which canbe distinguished from the remainder of the EFM signal, the other 564 EFMchannel bits being arranged as 14-bit EFM symbols. The synchronizationcode and the EFM symbols are always separated from one another by 3merging bits. The available EFM symbols are divided into 24 datasymbols, each representing 8 bits of the non encoded signal, 8 paritysymbols for the purpose of error correction, and one control symbolrepresenting 8 control bits. The 8 bits represented by each EFM controlsymbol are designated P, Q, R, S, T, U, V, W bits, each having a fixedbit position. The 16 bits of the EFM control symbols in the first twoEFM frames of each subcode frame form a subcode-synchronization signalindicating the beginning of the subcode frame. The remaining 96 Q bitsof the 96 residual EFM frames constitute the subcode Q-channel. Of thesebits, 24 bits are used to indicate an absolute time code. This absolutetime code indicates the time which has elapsed from the beginning of theEFM signal. This time is expressed in minutes (8 bits), second (8 bits)and subcode frames (8 bits).

Further it is to be noted that the EFM signal code is d.c. free, whichmeans that the EFM frequency spectrum exhibits hardly any frequencycomponents in the frequency range below 100 kHz.

FIG. 1 represents a record carrier 1, FIG. 1a being a plan view, FIG. 1billustrates a small portion of a sectional view taken on the line b--b,and FIG. 1c and FIG. 1d being plan views showing but a portion 2 of afirst and a second illustrative embodiment of the record carrier 1 butto a highly enlarged scale. The information carrier 1 has a servo track4, which is constituted, for example, by a preformed groove or ridge.The servo track 4 is intended for recording an information signal. Forthe purpose of recording, the record carrier 1 comprises a recordinglayer 6 which is deposited on the transparent substrate 5 and which iscovered with a protective coating 7. The recording layer 6 is made of amaterial which, if exposed to suitable radiation, is subjected to anoptically detectable change. Such a layer may be, for example, a thinlayer of a metal such as tellurium by exposure to laser radiation ofsufficiently high intensity this metal layer can be melted locally, sothat this layer is locally given another reflection coefficient. Whenthe servo track 4 is scanned by a radiation beam whose intensity ismodulated in conformity with the information to be recorded, aninformation pattern of optically detectable recording marks is obtainedwhich pattern is representative of the information.

The layer 6 may alternatively include different radiation-sensitivematerials, for example magneto-optical materials or materials which uponheating are subject to a structural change, for example from amorphousto crystalline or conversely. A survey of such materials is given in thebook "Principles of optical disc systems", Adam Hilgar Ltd., Bristol andBoston, pages 210-227.

A radiation beam aimed at the record carrier 1 for recording theinformation can be made to coincide accurately with the servo track 4 bymeans of the servo track, that is the position of the radiation beam ina radial direction can be controlled through a servo system utilizingthe radiation reflected from the record carrier 1. The measurementsystem for measuring the radial position of the radiation spot on therecord carrier may correspond to one of the systems as 25 described inthe aforementioned book "Principle of optical disc systems".

In order to determine the position of the track portion being scannedrelative to the beginning of the servo track a position informationsignal is recorded by means of a preformed track modulation, suitably inthe form of a sinusoidal track wobble as shown in FIG. 1c.

However, other track modulations such as, for example track-widthmodulation (FIG. 1d), are also suitable. Since a track wobble is verysimple to provide during the manufacture of the record carrier, a trackmodulation in the form of a track wobble is to be preferred.

It is to be noted that in FIG. 1, the track modulation has beenexaggerated greatly. In reality, a wobble having an amplitude of 30×10⁻⁹meters in the case of a track width of approximately 10⁻⁶ meters isfound to be adequate for a reliable detection of the scanning-beammodulation. A small amplitude of the wobble has the advantage that thedistance between adjacent servo tracks can be small.

An attractive track modulation is where the track-modulation frequencyis modulated in conformity with the position-information signal.

FIG. 2 demonstrates an example of a suitable position-information signalcomprising position-code signals 12 which alternate withposition-synchronization signals 11. Each position-code signal 12 maycomprise a biphase-mark modulated signal having a length of 76 channelbits, which signal represents a position-information code of 38 codebits. In a biphase-mark modulated signal, each code bit is representedby two successive channel bits. Each code of a first logic value, in thepresent example "0", is represented by two bits of the same logic value.The other logic value ("1") is represented by two channel bits ofdifferent logic values. Moreover, the logic value of the biphase-markmodulated signal changes after every pair of channel bits (see FIG. 2),so that the maximum number of successive bits of the same logic value istwo at the most. The position-synchronization signals 11 are selected insuch a way that they can be distinguished from the position-docesignals. This is achieved by selecting the maximum number of successivebits of the same logic value in the position-synchronization signals tobe equal to three. The position-information signal shown in FIG. 2 has afrequency spectrum which exhibits hardly any low-frequency components.The advantage of this will be explained hereinafter.

As stated in the foregoing, the position-information signal represents a38-bit position-information code. The 38-bit position-information codemay comprise a time code indicating the time needed to cover thedistance from the beginning of the track to the position where theposition-information signal is situated during scanning at nominalscanning velocity. Such a position-information code may comprise, forexample, a number of successive bytes, as used for example in recordingEFM modulated information on CD-Audio and CD-ROM discs. FIG. 3represents a position-information code which is similar to the absolutetime code employed in the case of CD-Audio ad CD-ROM and which comprisesa first BCD-encoded portion 13 indicating the time in minutes, a secondBCD-encoded portion 14 indicating the time in seconds, a thirdBCD-encoded portion 15 indicating a subcode-frame number, and a fourthportion 16 comprising a plurality of parity bits for the purpose oferror detection. Such a position-information code for indicating theposition in the servo track 4 is of advantage if an EFM-signal modulatedin conformity with the CD-Audio or CD-ROM standard is to be recorded. Inthat case, the absolute time codes present in the subcode Q-channel areof the same type as the position-information code represented by thetrack modulation.

In the case of a record carrier intended for recording EFM modulatedsignals in conformity with the CD-Audio or the CD-ROM standard, it isadvantageous that for a customary scanning velocity (1.2-1.4 m/s) theaverage frequency of the intensity modulation produced in the scanningbeam by the track modulation is 22.05 kHz. This means that the averageperiod of the track modulation should be between 54×10⁻⁶ meters and61×10⁻⁶ meters. In this technique, the record carrier velocity iscontrolled very simply by comparing the phase of the detected trackmodulation with the phase of a reference signal of a frequency which isderived simply by frequency division from the 4.3218 NHz frequency(which is the bit rate of the EFM signal, which is required anyway forrecording an EFM signal. Moreover, the frequency of the track modulationis situated outside the frequency band required for recording the EFMsignal, so that the EFM signal and the position-information signalhardly interact with each other during reading. In addition, thisfrequency is not within the frequency band of the tracking system, sothat the tracking is hardly affected by the track modulation.

If the channel-bit rate of the position-information signal is selectedto be 6300 Hz, the number of position-information codes which can beread is 75 per second, which is exactly the same as the number ofabsolute time codes per second of the EFM signal to be recorded. Ifduring recording the phase of the subcode-synchronization signal, whichindicates the beginning of the absolute time code, is locked to thephase of the position-synchronization signals represented by the trackmodulation, the absolute time indicated by the position-information coderemains in synchronism with the absolute time codes in the recorded EFMsignal.

FIG. 11a shows the position of the recorded subcode-synchronizationsignals relative to the track portions modulated in conformity with theposition-synchronization signals 11 if during recording the phaserelationship between the position-synchronization signal and thesubcode-synchronization signal is maintained constant .The servo-trackportions modulated in conformity with the position-synchronizationsignals 11 bear the reference numeral 140. The positions in which thesubcode-synchronization signals are recorded are indicated by the arrows141. As will be evident from FIG. 11a, the time indicated by theposition-information code remains in synchronism with the time indicatedby the absolute time code. If at the beginning of a recording theinitial value of the absolute time code is adapted to the positioninformation code, the track position indicated by the absolute time codewill always be equal to the track position indicated by theposition-information code. This has the advantage that for locatingspecific portions of the recorded signal both the absolute time code andthe position-information code may be used.

If as is indicated in FIG. 11b, the track positions 141, in which thesubcode-synchronization code is recorded, coincide with the trackportions 140 which are modulated in conformity with theposition-information signals, the difference between the track positionsrepresented by the position-information code and the absolute time codewill be minimal. Therefore, it is then advisable to minimize the phasedifference between the position-synchronization signals and thesubcode-synchronization signals during recording.

During reading of an EFM signal, the EFM channel clock is recovered fromthe signal being read. When a recorded EFM signal is read, the EFMchannel clock should therefore be available as soon as the first subcodeframe with useful information is read. This can be achieved, forexample, by adding one or more EFM blocks with dummy information at thebeginning of the EFM signal. This method is particularly suitable forrecording an EFM signal in a completely blank servo track.

However, if the EFM signal is to be recorded contiguously with apreviously recorded EFM signal, it is preferred to make the position inthe servo track 4 where the recording of the new EFM signal is to begincoincide substantially with the position where the recording of thepreviously recorded EFM signal has ceased. As in practice, the accuracywith which the beginning and end can be positioned is of the order ofmagnitude of a few EFM frames, either a small blank track portion willbe left between the track portions in which the signals are recorded orthe first and the second signal will overlap one another.

Such an overlapping or blank track portion results in the channel clockrecovery being disturbed. Therefore, it is preferred to select theboundary 144 between two recorded EFM signals 142 and 143 in such a waythat it is situated in an area between track portions 140, as isindicated in FIG. 11c. The portion from the boundary 144 up to thebeginning of the first subcode frame containing useful information isthen sufficiently long to restore the channel clock recovery before thebeginning of the first subcode frame containing useful information isreached. Preferably the position of the boundary 141 is selected to besituated before the center between the track portions 140a and 140b,because in that case a comparatively long time is available in which thechannel-clock recovery can be restored. However, the boundary 144 shouldbe situated sufficiently far from the end of the last subcode framecontaining useful information of the recorded EFM signal 112 (this endcorresponds to position 141a), in order to prevent that the lastcomplete subcode frame of the EFM signal 142 from being overwritten and,consequently, the last part of the information in the last subcode frameof the EFM signal 142 from being destroyed as a result of inaccuraciesin positioning of the beginning of the recording of the EFM signal 143.Apart from the destruction of recorded information, such an overlap alsoresults in the absolute time code belonging to the last subcode frameand the subcode-synchronization signal end of the subcode frame nolonger being read reliably. Since the absolute time code andsubcode-synchronization signals are used for controlling the readprocess, it is desirable that the number of non-readablesubcode-synchronization signals and absolute time-code signals isminimal. It will be evident that the recorded information of the EFMsignal 142 between position 141a and the boundary 144 cannot be readreliably. Therefore, it is also preferred to record dummy information,for example EFM pause-code signals in this part.

FIG. 4 illustrates recording and read apparatus 50 in accordance withthe invention wherein an EFM signal is recorded so that the positionsynchronization signals 11 represented by the track modulation remain insynchronism with the subcode-synchronization signals in the recorded EFMmodulated signal. The device 50 comprises a drive motor 51 for rotatingthe record carrier 1 about an axis 52. An optical read/write head 53 ofa customary type is arranged opposite the rotating record carrier 1. Theread/write head 53 has a laser for generating a radiation beam 55 whichis focused to form a tiny scanning spot on the record carrier 1.

The read/write head 53 can be operated in two modes, namely: a firstmode (read mode), in which the laser produces a radiation beam of aconstant intensity inadequate to bring about the optically detectablechange in the recording layer 6, and a second mode (recording mode, inwhich the radiation beam 55 is modulated depending on an informationsignal to be recorded in order to form a pattern of recording markshaving modified optical properties and corresponding to the informationsignal Vi in the recording layer 6 at the location of the servo track 4.

The recording and read apparatus 50 has tracking of a customary type,which keeps the scanning spot produced by the radiation beam 55 centeredon the servo track 4. As the servo track 1 is scanned the reflectedradiation beam 55 is modulated by the track modulation. By means of asuitable optical detector, the read/write head 53 detects the modulationof the reflected beam and produces a detection signal Vd representingthe detected modulation.

By means of a band-pass filter 56 having a mid frequency of 22.05 kHz,the frequency component modulated in conformity with theposition-information signal and produced by the track modulation isextracted from the detection signal. By means of an edge-restoringcircuit, for example a level controlled monostable 57, the output signalof the filter 56 is converted into a binary signal, which is applied toa frequency divider 59 via an EXCLUSIVE-OR gate 58. The output of thefrequency divider 59 is connected to one of the inputs of a phasedetector 60. A 22.05 kHz reference signal generated by aclock-generation circuit 63 is applied to a frequency divider 62 via anEXCLUSIVE-OR gate 61. The output of the frequency divider 62 isconnected to the other input of the phase detector 60. A signal which isindicative of the phase difference, determined by the phase detector 60between the signals on the two inputs is applied to an energizing(controller) circuit 61 for generating an energizing signal for thedrive motor 51. The feedback control circuit thus formed constitutes aphase-locked-loop velocity control system, which minimizes the detectedphase difference which is a measure of the velocity deviation.

The bandwidth of the phase-locked-loop velocity control system is small(generally of the order of magnitude of 100 Hz) in comparison with thebit rate 6300 Hz of the position-information signal. Moreover, theposition-information signal with which the frequency of the trackmodulation has been modulated does not contain any low-frequencycomponents, so that this FM modulation does not influence the velocitycontrol, the scanning velocity thus being maintained constant at a valuefor which the average frequency of the frequency components produced inthe detection signal Vd by the track modulation is maintained at 22.05kHz, which means that the scanning velocity is maintained at a constantvalue between 1.2 and 1.4 meters per second.

For the purpose of recording, the apparatus 50 comprises an EFMmodulation circuit 64 of a customary type, which circuit converts theapplied information into a signal Vi modulated in conformity with theCD-ROM or CD-Audio standard. The EFM signal Vi is applied to thewrite/read head via a suitable modulation circuit 71b, which convertsthe EFM signal into a sequence of pulses, in such a way that a patternof recording marks corresponding to the EFM signal Vi is recorded in theservo track 4. A suitable modulation circuit 71b is known, inter aliafrom U.S. Pat. No. 4,473,829. The EFM modulator is controlled by acontrol signal of a frequency equal to the EFM bit rate of 4.3218 MHz.The control signal is generated by the clock-generation circuit 63. The22.05 kHz reference signal, which is also generated by theclock-generation circuit 63, is derived from the 4.3218 MHz signal byfrequency division, so that a fixed phase-relationship is establishedbetween the control signal of the EFM modulator 61 and the 22.05 kHzreference signal. Since the control signal for the EFM modulator isphase-locked to the 22.05 kHz reference signal, the detection signal Vdis also phase-locked to this 22.05 kHz reference signal, so that theabsolute time codes generated by the EFM modulator remain in synchronismwith the position-information codes represented by the track modulationof the servo track 4 being scanned. However, if the record carrier 1exhibits flaws, for example scratches dropouts etc., it is found thatthis may give rise to an increasing phase difference between theposition-code signals and the absolute time codes.

In order to preclude this increasing phase difference, the phasedifference between the subcode-synchronization signals generated by theEFM modulator 64 and the position-synchronization signals being read isdetermined and the scanning velocity is corrected depending on the phasedifference thus determined. For this purpose a demodulation circuit 65is used Which extracts the position synchronization signals and theposition-code signals from the output signal of the filter 56 and,moreover, recovers the position-information codes from the position-codesignals.

The demodulation circuit 65, to be described in detail hereinafter,applies the position-information codes to a microcomputer 67 of acustomary type via a bus 66. Moreover, the demodulation circuit 65supplies a detection pulse Vsync via a signal line 68, which pulseindicates the instant at which a position synchronization signal is beendetected. The EFM modulator 64 comprises customary means for generatingthe subcode signals and for combining the subcode signals with the otherEFM information. The absolute time codes can be generated by means of acounter 69 and can be applied to the EFM modulator 64 via the bus 69a.The count of the counter 69 is incremented in response to control pulseshaving a frequency of 75 Hz. The control pulses for the counter 69 arederived from the 4.3218 MHz control signal by frequency division bymeans of the EFM modulator and are applied to the count input of thecounter 69 via a line 72a.

The EFM modulator 64 moreover generates the signal Vsub which indicatesthe instant at which the subcode-synchronization signal is generated.The signal Vsub is applied to the microcomputer 67 via a signal line 70.The counter 69 comprises inputs for setting the count to a value appliedvia these inputs. The inputs for setting the count are connected to themicrocomputer 67 via a bus 71. It is to be noted that it is alsopossible to include the counter 69 in the microcomputer 61.

The microcomputer 67 is loaded with a program to position the read/writehead 53 opposite the desired track prior to recording. The position ofthe read/write head 53 relative to the desired track is determined bymeans of the position-information codes generated by the demodulationcircuit 65 and the read/write head 53 is moved in a radial directionwhich depends on the position thus determined until the read/write headhas reached the desired position. For moving the read/write head 53 thedevice comprises the customary means for moving the read/write head 53in a radial direction, for example a motor 76 controlled by themicrocomputer 67 and a spindle 77. As soon as the desired track portionis reached the initial count of the counter 69 is adjusted to set theinitial value for the absolute time code to the value corresponding tothe position-information code of the track portion being scanned.Subsequently the read/write head 53 is set to the write mode by themicrocomputer 67 via a signal line 71a and the EFM modulator 64 isactivated via a signal line 72, to start the recording, the recording ofthe absolute time codes in the EFM signal being maintained insynchronism, in the same way as described hereinbefore, with theposition-code signal represented by the track modulation at therecording position. This has the advantage that the recorded absolutetime codes always correspond to the position-code signals represented bytrack modulation at the track portion in which the absolute time codesare recorded. This is of particular advantage if different informationsignals have been recorded after one another, because the absolutetime-code signals do not exhibit any abrupt changes at the transitionbetween two successively recorded EFM signals. Thus, in order to locatespecific portions of the recorded information signals it is possible toutilize both the absolute time codes recorded together with theinformation signal and the position-code signals represented by thetrack modulation, which yields a highly flexible retrieval system.

By way of illustration FIG. 7 shows a pattern of recording marks 100formed when the EFM signal Vi is recorded in the servo track 4. It is tobe noted again that the bandwidth of the tracking control issubstantially smaller than the frequency of the scanning-beam modulationcaused by the track modulation (in the present case in the form of atrack wobble), so that the tracking control does not respond to trackingerrors caused by the track undulation. Therefore, the scanning beam willnot exactly follow the track but will follow a straight path which isrepresentative of the average position of the center of the servo track4. However, the amplitude of the track wobble is small, suitably of theorder of magnitude of 30×10⁻⁹ meters (=60×10⁻⁹ meters peak to peak), incomparison with the track width, which is of the order of magnitude 10⁻⁶meters, so that the pattern of recording marks 100 is alwayssubstantially the centered relative to the servo track 4. It is to benoted that for the sake of clarity a rectangular track wobble is shown.However, in practice it is preferred to use a sinusoidal track wobble,because this minimizes the number of high-frequency components in themodulation of the scanning beam 55 produced by the track modulation, sothat the EFM signal being read is affected to a minimal extent.

During recording the microcomputer 67 performs a program to derive fromthe signals Vsync and Vsub applied via the signal lines 68 and 70 derivethe time interval between the instant at which a synchronization signalis detected in the track portion being scanned and the instant at whichthe subcode-synchronization signal is generated. As long as the positionsynchronization signal leads the subcode-synchronization signalgeneration by more than a predetermined threshold value themicrocomputer 67 supplies one or more additional pulses to the divider59 via the signal line 73 and the EXCLUSIVE-OR gate 58 after everysynchronization signal detection, which causes the phase differencedetected by the phase detector 60 to increase and which causes theenergizing circuit 61 to reduce the speed of the drive motor 53, so thatthe Phase difference between the detected position-synchronizationsignals and the generated subcode-synchronization signal decreases.

As long as the detected synchronization signal lags the generatedsubcode-synchronization signal by more than a predetermined thresholdvalue the microcomputer 67 applies additional pulses to the divider 62via a signal line 74 and the EXCLUSIVE-OR gate 61. This causes the phasedifference detected by the phase detector to decrease, as a result ofwhich the speed of the drive motor 53 increases and the phase differencebetween the detected position-synchronization signals and the generatedsubcode-synchronization signals decreases. In this way a permanentsynchronization between the two synchronizing signals is maintained. Itis to be noted that in principle it is also possible to adapt the writevelocity instead of the scanning velocity in order to maintain thedesired phase relationship. This is possible, for example, by adaptingthe frequency of the control signal of the EFM modulator 64 depending onthe detected phase difference.

FIG. 5 is a flowchart of a suitable program for maintaining thesynchronization The program comprises a step S1 in which the timeinterval T between the detection instant Td of the synchronizationsignal read and the generation instant To of the subcode-synchronizationsignal is determined in response to the signals Vsub and Vsync on thesignal lines 68 and 70. In step S2, it is ascertained whether the timeinterval T is greater than a predetermined threshold value Tmax. If itis greater, step S3, is carried out, in which an additional pulse isapplied to the counter 62. After step S3 step S1 is repeated.

However, if the time interval T thus determined is smaller than Tmax,step S2 is followed by step S4, in which it is ascertained whether thetime interval T is smaller than a minimum threshold value Tmin. If it issmaller, step S5 is performed, in which an additional pulse is appliedto the counter 59. After step S5, step S1 is repeated. If during step S4it is found that the time interval is not smaller than the thresholdvalue, no additional pulse is generated but the program proceeds withstep S1.

FIG. 12 provides a flow chart of a suitable program for themicrocomputer 67 for recording an EFM signal contiguously with apreviously recorded EFM signal. The program includes a step S10 in whichthe position-information code AB is determined, which code indicates theposition where the previously recorded information ends. This positioninformation code can be stored in the memory of the microcomputer 67,for example, after recording of the preceding signal. Moreover, in stepS10 the position-information code AE is derived from the number ofsubcode frames to be recorded, which code indicates the position wherethe recording should end. This information can be generated, forexample, by the storage medium in which the information to be recordedis stored and can be applied to the microcomputer 67. This storagemedium and the method of detecting the length of the signal to berecorded fall beyond the scope of the present invention and aretherefore not described any further. After step S10 step S11 isperformed, in which in conventional manner the read/write head 53 ispositioned opposite a track portion which precedes the point where therecording of the EFM signal should begin Control means suitable for thispurpose are described comprehensively inter alia in U.S. Pat. No.4,106,058.

Subsequently in step 11a the detection signal Vsync is awaited whichdetection signal is supplied by the demodulation circuit 65 via thesignal line 68 and indicates that a newly read position-information codeis applied to the bus 66. In step S12, this position information code isread in and in step S13 it is ascertained whether this read inposition-information code corresponds to the position information codeAB indicating the starting point of the recording. If this is not thecase, step S13 is followed by step S11a. The program loop of steps S11a,S12 and S13 is repeated until the read in position-information codecorresponds to the position-information code AB. After this, in stepS11, the initial value of the absolute time code in the counter 69 isset in conformity with the position information code AB. Subsequently,in step S15, the EFM modulator 61 is put into operation via the signalline 72.

In step S16 a waiting time Td is observed, which time corresponds to thedisplacement of the scanning spot over a distance corresponding to thedistance SW between the boundary 144 and the preceding track portion 140(see FIG. 11c). At the end of the waiting time, the position of thescanning spot in the servo track 4 corresponds to the desired startingposition of the recording and the read/write head 53 is set to the writemode during step S17, after which recording is started. Subsequently, instep S18 every following detection pulse Vsync is awaited and afterthis, in step S19, the detected position-information code is read in,upon which it is ascertained in step S20 whether the read-inposition-information code corresponds to the position-information codeAE indicating the end of the recording. In the case ofnon-correspondence the program proceeds with step S18 and in the case ofcorrespondence a waiting time Td is observed in step S21 beforeproceeding with step S22. In step S22 the read/write head 53 is againset to the read mode. Subsequently in step S23 the EFM modulator 61 isde-activated.

The above method of determining the track positions indicating thebeginning and the end of the recording utilizes the prerecordedposition-information codes. However, it is to be noted that it is notstrictly necessary to determine the position-information codes in orderto detect the beginning and end positions. For example, by countingprerecorded position-synchronization signals from the beginning of theservo track 4, it is also possible to detect the position of the trackportion being scanned.

FIG. 6 provides an illustrative embodiment of the demodulation circuit65 in detail. The demodulation circuit 65 comprises an FM demodulator80, which recovers the position-information signal from the outputsignal of the filter 56. A channel clock regeneration circuit 81regenerates the channel clock from the recovered position-informationsignal.

The position-information signal is further applied to a comparatorcircuit 82, which converts this signal into a binary signal which isapplied to an 8-bit shift register 83, which is controlled by thechannel clock. The parallel outputs of the shift register 83 are fed toa synchronization signal detector 84, which detects whether the bitpattern stored in the shift register corresponds to the positionsynchronization signal. The serial output of the shift register 83 isconnected to a biphase-mark demodulator 85 for the recovery of the codebit of the position-information code represented by the biphase-markmodulated position-code signal. The recovered code bits are applied to ashift register 86 which is controlled by a clock frequency equal to halfthe channel-clock frequency and which has a length equal to the numberof bits (38) of the position-code signal.

The shift register 86 comprises a first section 86a having a length of14 bits and a second section 86b having a length of 24 bits andfollowing the first section 86a.

The parallel outputs of the first section 86a and the second section 86bare fed to an error detection circuit 87. The parallel outputs of thesecond section 86b are fed to a parallel-in parallel-out register 88.

The position-information code is recovered as follows. As soon as thesynchronization signal detector 84 detects the presence of a bit patterncorresponding to the position synchronization signal in the shiftregister 83 a detection pulse is generated which is applied to a pulsedelay circuit 90 via a signal line 89. The circuit 90 delays thedetection pulse by a specific time corresponding to the processing timeof the biphase-mark modulator so that after the instant at which thedetection pulse from the signal line 68 appears on the output of thedelay circuit 90 a complete position-information code is present in theshift register 86. The delayed detection pulse on the output of thecircuit 90 is also applied to the load input of the register 88, so thatthe 24 bits representing the position-information code are loaded intothe register 88 in response to the delayed detection pulse. Theposition-information code loaded into the register 88 is available onthe output of the register 88, which outputs are connected to themicrocomputer 67 via the bus 66. The error detection circuit 87 is alsoactivated by the delayed detection pulses on the output of the circuit90, after which the detection circuit 87 detects whether the receivedposition-information code is reliable in conformity with a customarydetection criterion. An output signal which indicates whether theposition information is reliable is applied to the microcomputer 67 viaa signal line 91.

FIG. 8 depicts an illustrative embodiment of apparatus 181 formanufacturing a record carrier 1 in accordance with the invention.Apparatus 181 comprises a turntable 182 which is rotated by a drivemeans 183. The turntable 182 is adapted to support is disc-shapedcarrier 184, for example a flat glass disc provided with a radiationsensitive layer 185, for example, in the form of a photoresist.

A laser 186 produces a light beam 187 which is projected onto thelight-sensitive layer 185. The light beam 187 is first passed through adeflection device. The deflection device is of a type Which enables alight beam to be deflected very accurately within a narrow range. In thepresent example, the device is an acousto-optical modulator 190. Thedeflection device may also be formed by other devices, for example, amirror which is pivotable through a small angle or an electro-opticaldeflection device. The limits of the deflection range are indicated by abroken line in FIG. 8. The light beam 187 deflected by theacousto-optical modulator 190 is passed to an optical head 196. Theoptical head 196 comprises a mirror 197 and an objective 198 forfocusing the light beam onto the light-sensitive layer 185. The opticalhead 196 is radially movable relative to the rotating carrier 184 by anactuating device 199.

By means of the optical system described above the light 35 beam 187 isfocused to form a scanning spot 102 on the radiation-sensitive layer185, the position of this scanning spot 102 is dependent on thedeflection of the light beam 187 by the acousto-optical modulator 190and on the radial position the write head 196 relative to the carrier184. In the illustrated position of the optical head 196, the scanningspot 102 can be moved within a range B1 by means of the deflectiondevice 190. By the optical head 196, the scanning spot 102 can be movedthrough a range B2 for the indicated deflection.

The device 181 includes a control device 101, which may comprise forexample the system described in detail in Netherlands Patent Application8701418 which corresponds to U.S. Pat. No. 4,864,544, herewithincorporated by reference. By means of this control device 101, thespeed of the drive means 183 and the radial velocity of the actuatingdevice 199 are controlled in such a way that the light-sensitive layer185 is scanned with a constant scanning velocity along a spiral path bythe radiation beam 187. The device 181 further comprises a modulationcircuit 103 for generating a periodic drive signal whose frequency ismodulated in conformity with the position-information signal. Themodulation circuit 103 will be described in detail hereinafter. Thedrive signal generated by the modulation circuit 103 is applied to avoltage-controlled oscillator 101 which generates a periodic drivesignal for the acousto optical modulator 104, whose frequency issubstantially proportional to the signal level of the drive signal. Adeflection produced by the acousto-optical modulator 190 is proportionalto the frequency of the drive signal in such a way that the displacementof the scanning spot 102 is proportional to the signal level of thedrive signal. The modulation circuit 103, the voltage-controlledoscillator 101, and the acousto-optical modulator 190 are adapted to oneanother in such a way that the amplitude of the periodic radialexcursion of the scanning spot 102 is approximately 30×10⁻⁹ meters.Moreover, the modulation circuit 103 and the control circuit 101 areadapted to one another in such a way that the ratio between the averagefrequency of the drive signal and the scanning velocity of theradiation-sensitive layer 108 is situated between 22050/1.2 m⁻¹ and22050/1.4 m⁻¹, which means that in every period of the drive signal thedisplacement of the radiation-sensitive layer 185 relative to thescanning spot is between 54×10⁻⁶ meters and 64×10⁻⁶ meters.

After the layer 185 has been scanned as described in the foregoing, itis subjected to an etching process to remove the portions of the layer185 which have been exposed to the radiation beam 187 yielding a masterdisc in which a groove is formed which exhibits a periodic radial wobblewhose frequency is modulated in conformity with the position-informationsignal. From this master disc, replicas are made on which the recordinglayer 6 is deposited. In record carriers of the inscribable type thusobtained the part corresponding to that part of the master disc fromwhich the radiation-sensitive layer 185 has been removed is used as theservo track 4 (which may be either a groove or a ridge). A method ofmanufacturing a record carrier in which the servo track 4 corresponds tothat part of the master disc from which the radiation-sensitive layerhas been removed has the advantage of a very good reflection of theservo track 4 and hence a satisfactory signal-to noise ratio during readout of the record carrier. Indeed, the servo track 4 then corresponds tothe highly smooth surface of the carrier 184, which is generally made ofglass.

FIG. 9 provides an example of the modulation circuit 103 the modulationcircuit 103 comprises three cascaded cyclic 8-bit BCD counters 110, 111and 112. The counter 110 is an 8-bit counter and has a counting range of75. When its maximum count is reached the counter 110 supplied a clockpulse to the count input of the counter 111, which is employed asseconds counter. After its maximum count 59 is reached the counter 111supplies a clock pulse to the count input of the counter 112, whichserves as minutes counter. The counts of the counters 110, 111 and 112are applied to a circuit 116 via the parallel outputs of the countersand via the buses 113, 114 and 115 respectively to derive the fourteenparity bits for the purpose of error detection in a customary manner.

The modulation circuit 103 further comprises a 42-bit shift register 117divided into five successive sections 117a, . . . 117e. A bitcombination "1001" is applied to the four parallel inputs of the 4-bitsection 117a, which bit combination is converted into the positionsynchronization signal 11 in a manner to be described hereinafter duringthe biphase mark modulation. The sections 117b, 117c and 117d each havea length of 8 bits and the section 117e has a length of 14 bits. Thecount of the counter 111 is applied to the parallel inputs of section117b via the bus 115. The count of the counter 111 is applied to theparallel inputs of the section 117c via the bus 114. The count of thecounter 110 is transferred to the parallel inputs of the section 117dvia the bus 113. The fourteen parity bits generated by the circuit 116are applied to parallel inputs of section 117b via a bus 116a.

The serial output signal of the shift register is fed to a biphase-markmodulator 118. The output of the modulator 118 is applied to an FMmodulator 119. The circuit 103 further comprises a clock-generationcircuit 120 for generating the control signals for the counter 118, theshift register 117, the biphase-mark modulator 118 and the FM modulator119.

In the present example the radiation-sensitive layer 185 is scanned witha velocity corresponding to the nominal scanning velocity of EFMmodulated signals, (1.2-1.4 m/s) during manufacture of the master disc.The clock-generation circuit 120 then generates a 75-Hz clock signal 139for the counter 110, so that the counts of the counters 110, 111 and 112constantly indicate the time elapsed during scanning of the layer 18S.

Immediately after adaptation of the counts of the counters 110, 111 and112 the clock-generation circuit supplies a control signal 128 to theparallel load input of the shift register 117, causing the shiftregister to be loaded in conformity with the signals applied to theparallel inputs, namely the bit combination "1001", the counts of thecounters 110, 111 and 112, and the parity bits.

The bit pattern loaded into the shift register 117 is applied to thebiphase-mark modulator 118 via the serial output in synchronism with aclock signal 138 generated by the clock-generation circuit 120. Thefrequency of this clock signal 138 is 3150 Hz, so that the entire shiftregister is empty at the very instant at which it is reloaded via theparallel inputs.

The biphase-mark modulator 118 converts the 42 bits from the shiftregister into the 84 channel bits of the position-code signal. For thispurpose the modulator 118 comprises a clocked flip-flop 121 whose outputlogic level changes in response to a clock pulse on the clock input. Bymeans of a gate circuit, the clock signals 122 are derived from thesignals 123, 121, 125 and 126 generated by the clock-generation circuit120 and from the serial output signal 127 of the shift register 170 Theoutput signal 127 is applied to an input of a two-input AND gate 129.The signal 123 is applied to the other input of the AND gate 129. Theoutput signal of the AND gate 129 is applied to the clock input of theflip-flop 121 via an OR gate 130. The signals 125 and 126 are applied tothe inputs of the OR gate 131, whose output is connected to one of theinputs of a two-input AND gate 132. The output signal of the AND gate132 is also applied to the clock input of the flip-flop 121 via theOR-gate 130.

The signals 123 and 124 are two 180° phase-shifted pulse-shaped signals(see FIG. 10) of a frequency equal to the bit rate of the signal 127(=3150 HZ) from the shift register 117. The signals 125 and 126 arenegative pulses having a repetition rate of 75 Hz.

The phase of the signal 125 is such that the negative pulse of thesignal 125 coincides with the second pulse of the signal 124 afterreloading of the shift register 117. The negative pulse of the signal126 coincides with the fourth pulse of the signal 124 after reloading ofthe shift register 117.

The biphase-mark-modulated position-code signal 12 on the output of theflip-flop 121 is generated as follows. The pulses of the signal 124 aretransferred to the clock input of the flip-flop 121 via the AND gate 132and the OR gate 130, so that the logic value of the position-code signal12 changes in response to every pulse of the signal 124. Moreover, ifthe logic value of the signal 127 is "1" the pulse of the signal 123 istransferred to the clock input of the flip-flop 121 via the AND gates129 and 130, so that for every "1" bit an additional change of the logicsignal value is obtained. In principle, the synchronization signals aregenerated in a similar way. However, the application of the negativepulses of the signals 125 and 126 prevents the second and the fourthpulse of the signal 124 after reloading of the shift register from beingtransferred to the flip-flop 121, yielding a position-synchronizationsignal which can be distinguished from a biphase-mark-modulated signal.It is to be noted that this modulation method may lead to two differentsynchronization signals which are inverted relative to one another.

The position-information signal thus obtained on the output of the flipflop 121 is applied to the FM modulator 119, which is 35 suitably of atype with a fixed relationship between the frequencies generated on theoutput of the FM modulator and the bit rate of the position-informationsignal. When the scanning velocity control is not disturbed thesubcode-synchronization signals in the EFM signal remain in synchronismwith the position-synchronization signals 11 in the track 4 duringrecording of an EFM signal by means of apparatus 50. Disturbances in thevelocity control resulting from imperfections of the record carrier canbe compensated for by very small corrections, as already described withreference to FIG. 4.

In the FM modulator 119 shown in FIG. 9 an advantageous relationshipbetween the output frequencies and the bit rates of theposition-information signal is obtained. The FM modulator 119 comprisesa frequency divider 137 having a divisor "8". Depending on the logicvalue of the position-information signal a clock signal 134 having afrequency of (27).(6300) Hz or a clock signal 135 having; a frequency of(29).(6300) Hz is applied to the frequency divider 137. For this purposethe FM modulator 119 comprises a conventional multiplex circuit 136depending on the logic value of the position-information signal thefrequency on the output 133 of the FM modulator is ##EQU1##

Since the frequency of the signals 134 and 135 are integral multiples ofthe channel-bit rate of the position-information signal the length ofone channel bit corresponds to an integral number of periods of theclock signals 134 and 135 which means that the phase steps in FMmodulation are minimal.

Moreover, it is to be noted that on account of the d.c. component of theposition-information signal the average frequency of the FM-modulatedsignal is exactly equal to the 22.05 kHz, which means that the velocitycontrol is influenced to a negligible extent by the FM modulation.

Moreover, it is to be noted that for the FM modulator other FMmodulators can be used than the modulator 119 shown in FIG. 9, forexample a conventional CPFSK modulator (CPFSK=Continuous Phase FrequencyShift Keying). Such CPFSK modulators are described inter alia in A.Bruce Carlson: "Communication Systems", McGraw Hill, page 519 ff.

Moreover, it is preferred to utilize an FM modulator with a sinusoidaloutput signal. With the FM modulator 119 shown in FIG. 9 this can beachieved, for example, by arranging a band-pass filter between theoutput of the divider 117 and the output of the modulator 119. Further,it is to be noted that the frequency swing is suitably of the order ofmagnitude of 1 kHz.

Finally, it is to be noted that the scope of the invention is notlimited to the illustrative embodiments described herein. For example,in the embodiments described the frequency spectrum of theposition-information signal exhibits substantially no overlap with thefrequency spectrum of the signal to be recorded. However, theposition-information signal recorded by means of the preformed trackmodulation can, in that case, always be distinguished from thesubsequently recorded information signal. In the case of magneto-opticalrecording, the frequency spectra of the prerecorded position-informationsignal and the subsequently recorded information signal may overlap oneanother. Indeed, during scanning with a radiation beam the trackmodulation results in an intensity modulation of the radiation beam,whilst the information pattern formed by magnetic domains modulates thedirection of polarization (Kerr-effect) of the reflected radiation beamindependently of the intensity modulation. In the embodiments describedin the foregoing, the scanning beam is modulated depending on theinformation to be recorded. In the case of recording on magneto-opticalrecord carriers, it is also possible to modulate the magnetic fieldinstead of the scanning beam.

What is claimed is:
 1. An optically readable and inscribable recordcarrier comprising: a recording layer for recording an informationpattern of optically detectable recording marks, the record carrierhaving a servo track wherein a portion for information recordingincludes a periodic track modulation different from the informationpattern, the periodic track modulation having a modulation frequencyindicative of a position-information signal comprising position-codesignals alternating with position-synchronization signals.
 2. Anoptically readable inscribable record carrier as claimed in claim 1,characterized in that the position-code signals arebiphase-mark-modulated signals and the position-synchronization signalshave signal waveforms different from the biphase-mark-modulated signal.3. A record carrier as claimed in any one of the preceding claims,characterized in that the servo track has a width that is between0.4×10⁻⁶ meters and 1.25×10⁻⁶ meters, and the periodic track modulationbeing a track wobble having an amplitude which is substantially equal to30×10⁻⁹ meters.
 4. A record carrier as in either claim 1 or claim 2,characterized in that the periodic tack modulation has a period between54×10⁻⁶ meters, and 64×10⁻⁶ meters and a distance between startingpositions of the track portions includes the position-synchronizationsignal corresponding to 294 times an average of the period of the trackmodulation.
 5. A record carrier as claimed in any of the claims 1 or 2,characterized in that the position-code signal is indicative of elapsedtime at a nominal scanning velocity to cover a distance between abeginning of the track and a position where the track provides trackmodulation corresponding to the position where the track provides trackmodulation corresponding to the position-code signal.
 6. A recordcarrier as claimed in claim 5, characterized in that the position-codesignal is modulated in conformity with a position-information code whichcomprises at least a portion similar to an absolute-time code containedin an EFM-modulated signal in conformity with the CD-standard.
 7. Anapparatus for manufacturing a record carrier as claimed in any one ofclaims 1 or 2, comprising and optical system for scanning aradiation-sensitive layer of a carrier along a path corresponding to theservo track to be formed and a deflection device for deflecting theradiation beam in such a way that the position of incidence of the beamon the radiation-sensitive layer is deflected in a directionperpendicular to the scanning direction corresponding to a controlsignal applied to the deflection device, characterized in that theapparatus comprises a signal generator for generating a periodic signalas the control signal and the control signal having a frequency that ismodulated by the position-information signal.
 8. An apparatus as claimedin claim 7, characterized in that the signal generator comprises amodulator for converting a position-information code into theposition-information signal and an FM modulator for modulating thefrequency of the control signal in conformity with theposition-information signal.
 9. An apparatus as claimed in claim 8,characterized in that the modulator comprises a biphase-mark modulatorand means for generating synchronization signals of a signal waveformdiffering from a biphase-mark-modulated signal.
 10. An apparatus asclaimed in claim 8, characterized in that the apparatus comprises meansfor generating a position-information information code indicatingelapsed time at a nominal scanning velocity of the record carrier tocover the distance between a starting position of a servo track and theposition where the position-information code is recorded as a trackmodulation.
 11. An apparatus as claimed in claim 10, characterized inthat the apparatus comprises means for generating a position-informationcode similar to the absolute-time code contained in an EFM-modulatedsignal in conformity with the CD-standard.
 12. An apparatus as claimedin claim 7 characterized in that the apparatus comprises means formaintaining a ratio between a scanning velocity of the radiationsensitive layer and an average frequency of the control signal, theaverage frequency having a period between 54×10⁻⁶ meters and 64×10⁻⁶meters, and the signal generator generating position-synchronizationsignals of a frequency equal to 294 times the average frequency of thecontrol signal.
 13. An apparatus as claimed in claim 7 characterized inthat the signal generator produces a control signal having an amplitudeof a value for which the excursion of the position of incidence on theradiation-sensitive, which excursion corresponds to the control signal,has an amplitude of substantially 30.10⁻⁹ meters.
 14. An apparatus forrecording information on a record carrier as claimed in any one of theclaims 1 or 2, the apparatus comprising write means for forming apattern of recording marks indicative of information in the servo track,the write means including scanning means for scanning the servo trackwith a radiation beam, the radiation beam reflected or transmitted bythe record carrier being modulated by the track modulation, a detectorfor detecting the reflected or transmitted radiation beam, and a circuitfor deriving a clock signal of a frequency dictated by the trackmodulation from the radiation detected by the detector, the recordingapparatus comprising and RM demodulator for recovering theposition-information signal from the clock signal and means forseparating the position-code signals and the synchronization-signals.15. An apparatus as claimed in claim 14, characterized in that theapparatus comprises a biphase mark demodulator for recovering theposition codes from the position-code signals.
 16. An apparatus forreading a record carrier as claimed in any one of the claims 1 or 2, onwhich record carrier an information signal is recorded as a pattern ofrecording marks in the servo track, the apparatus comprising a scanningdevice for scanning the servo track with substantially constant velocityby a radiation beam, the radiation beam reflected or transmitted by therecord carrier being modulated by the track modulation and the patternof recording marks, a detector for detecting the reflected ortransmitted radiation beam, a circuit for deriving an information signalrepresenting the recorded information from the radiation beam detectedby the detector, and a circuit for deriving a clock signal whosefrequency is determined by the track modulation from the radiation beamdetected by the detector, and the apparatus comprises an FM demodulationcircuit for recovering the position-information signal from the clocksignal and means for separating the position-code signals and thesynchronization signals.
 17. An apparatus as claimed in claim 16,characterized in that the apparatus comprises a biphase-mark demodulatorfor recovering the position code from the position-code signals.